Cardiff has lots of shops, touristy bits and maybe some Rugby on depending on when you go. Probably your top bet for nightlife, clubs anyway.
Swansea has The Mumbles, a beautiful part of the world with lovely walking, cycling or beach possibilities.
Barns and places with big gardens (unless you find a hostel or bunkhouse near a beach or field or something) may be a bit out of the way so bear in mind public transport unless any of you drive and taxi fares to get all the way back.
Tenby is lovely, boat trips are great and they do ones to coves with a BBQ in the summer and there is an old island to explore, the beach and, of course, the beach. Maybe some watersports or a fishing trip. You might find something more rustic round there or a bit along the coast. Quite a few pubs there.
Aberystwyth is a nice little town. It's a university town so although small it's packed with bars and a good amount of young bars as well as more traditional bars (watch the very traditional, if you don't speak Welsh fluently best to avoid them). There are the sand dunes up at Ynyslas (http://www.visitmidwales.co.uk/Borth-Ynyslas-National-Nature-Reserve/details/?dms=13&venue=1012220) and some lovely drives down the coast or up to Snowdonia which is a stunning part of the world. You could go climb Snowdon (or take the train up/down).
A fair few bunkhouses round Snowdonia but not a huge amount of nightlife round there.
Lake Bala is another possible, lots of watersports on offer.
http://www.visitbala.org/
I'm not sure about the nightlife though.